Microstructure And Tensile And Creep Behavior Of GH4169 Fabricated By Laser Powder Bed Fusion

Laser Powder Bed Fusion(LPBF)technology has been widely concerned in key engineering areas such as aerospace,rail transit and energy in recent years,because it can realize the forming of geometrically complex parts based on computer-aided design.However,the unique layer-layer stacking process makes the microstructure and mechanical properties of the LPBF-fabricated alloy different from those of traditional wrought and casting alloys.More importantly,the mechanical properties of the LPBF-fabricated parts will change with the variation of the process parameters and part geometry.Therefore,understanding and clarifying the relationship between processing-structure-property is one of the key scientific problems to realize the industrial application of the LPBF-fabricated parts and ensure their reliable service.Furthermore,the mechanical properties under elevated temperature are one of the most important indexes to evaluate the service reliability of nickel-base superalloys for aerospace applications.Therefore,in this thesis,LPBF-fabricated GH4169 using in the blades was chosen as the research object,the relationship between processing-structure-property was systematically investigated based on the miniature specimen,focusing on the influence of building orientation,height,and thickness on creep properties of the LPBF-fabricated GH4169 at 650℃.The results about the tensile properties and microstructure of LPBF-fabricated GH4169 under stripe 67° and chessboard scanning strategies show that compared with the stripe 67° specimen,the chessboard specimen has higher tensile strengths and more evident tensile anisotropy,which is mainly attributed to the higher residual compressive stress along the direction normal to the building direction.Furthermore,the chessboard specimen has more excellent strain hardening ability due to the heterogeneous grain structure.Through regulation of heat treatment conditions,it was found that compared with the as-built specimen,the yield strength of the heat-treated specimens increases resulting from the precipitation of the γ" strengthened phase and the release of the residual tensile stress along building direction.Meanwhile,the strain hardening ability of specimens in different states differs widely.The solution+aging specimen exhibits a superior strain hardening ability.This is mainly because that the uniformly distributed shear-resistant needle-shaped δ phase at dendritic boundaries will interact with the dislocations to leave a larger number of Orowan loops during the plastic strain,and effectively improves the dislocation storage rate.Combined with experimental observation and theoretical calculation,the internal mechanism of strain hardening behavior of the as-built,solid solution+aging and homogenization+aging alloy was revealed,and a strain hardening model suitable for LPBF-fabricated GH4169 with different microstructure was established.The results about the building orientation-dependent tensile and stress rupture properties of LPBF-fabricated GH4169 at elevated temperature show that the specimen with the building direction parallel to the loading direction has lower tensile strength but longer stress rupture life at 650℃.The former is mainly attributed to the weak deformation resistance along the direction parallel to the building direction for the columnar grain epitaxially grown along the building direction.The latter is mainly since when the building direction and the loading direction are parallel,the content of grain boundaries perpendicular to the loading direction in the specimen can be reduced,effectively delaying the void nucleation,and the columnar grain boundaries between adjacent grains can hinder the crack propagation,effectively reduce the crack growth rate,and thus prolong the stress rupture life.The results about the building height-dependent microstructures and creep properties of LPBF-fabricated GH4169 at elevated temperature show that the creep life increases gradually with the decrease of the building height at 650 ℃,showing an obvious building height dependence.Based on the creep model dominated by dislocation slip and the creep void nucleation theory,it was shown that under the same heat treatment condition,the decrease in dislocation density and grain boundary volume fraction caused by the increase in recrystallization degree of the top to bottom specimens can effectively delay the creep rate and reduce the potential nucleation sites of creep voids,thus improving the creep properties of the LPBF-fabricated GH4169.And the increase in recrystallization degree of the top to bottom specimen results from the decrease in the dislocation density and the increase in the texture intensity along the building direction.The results about the building thickness-dependent micro structures and creep properties of LPBF-fabricated GH4169 at elevated temperature show that the creep life of the LPBF-fabricated GH4169 increases gradually with the building thickness increasing at 650℃,showing an obvious building thickness dependence.This is mainly due to under the same heat treatment condition,the thicker specimen has a lower density of Laves phase along grain boundaries,more γ" phase and smaller phase spacing within the grain resulting from the difference in initial microstructures between the specimens with different building thicknesses.While the γ" phase can effectively hinder the dislocation movement,and the creep rate decreases significantly with the decrease in the phase spacing.At the same time,based on the theory of creep void nucleation,the decrease of Laves phase density at grain boundaries can effectively reduce the nucleation rate of creep void and delay the creep cracking.Therefore,the thicker specimen has higher creep life.In addition,the specimens with different building thicknesses show obvious creep anisotropy due to<001>texture along the building direction and elongated grains with long axes parallel to the building direction.The present findings reveal that the influences of the building orientation,height and thickness-dependent creep performance on the overall service reliability should be considered comprehensively in the subsequent geometrical design of the LPBF-fabricated complex structural components.The research findings may have important theoretical significance and practical reference value for optimizing the creep properties of GH4169 at elevated temperature and exploring customized strategies for the LPBF-fabricated key engineering components.